The adenine-3 (ad-3) foward mutation system in Neurospora crassa is one of the best systems for studying mutagenesis in eukaryotic microorganisms. With this test system, recessive lethal mutations resulting from either chromosome deletions or point mutations can be recovered at the ad-3 region. The limitations of this test system are that one cannot screen for the mutagenicity of a large number of chemicals in a short period of time and that not every laboratory is equipped to use it. It is necessary to develop simple test systems in Neurospora so that both reverse and forward mutations caused by environmental agents can easily be detected. In our previous studies we have developed a spot test system in Neurospora using two ad-3A testers. Further studies are being carried out to determine: (1) whether these two testers can be used in plate and suspension tests, (2) whether mutagenicity of procarcinogens can be detected in the vegetative cultures of these testers, (3) whether mutants carrying double UV-sensitive markers are more sensitive to mutagens, and (4) whether mutants carrying a cell wall defective genetic marker are more sensitive to mutagens. Studies are also being carried out to develop both a para-fluorophenylalanine (PFPA) and a cyclohexamide (CH) resistant forward mutation systems.